For the generation of a qualitative high value paper web it is necessary for the material suspension which leaves the outflow slot which, for example, has a water component of over 90%, to be present in a form as homogenous as possible. Of importance in this respect is above all a uniform density of the entire paper web.
In order to avoid flock formation within the material suspension and in order to break up flock accumulations which have already arisen, two different types of headboxes are used which have become known under the terms hydraulic headboxes and apertured roller headboxes.
In a hydraulic headbox the material suspension which is supplied via a distributor is fed to a stationary turbulence generator. For example, it can be directed at an elevated speed through a plurality of outflow channels which have step-wise increasing diameters. At the steps the material suspension undergoes a sudden volume increase. The tearing apart of the flocks is brought about as a consequence of the shearing forces which occur at the steps.
Hydraulic headboxes have the advantage that, above all with long fibres and high material densities, a very homogenous consistency can be achieved of the material suspension leaving the outflow gap, they have however the disadvantage that with short fibres and low material densities inhomogeneities increase, in particular in the form of harder flocks, which leads to a granular, seedy consistency of the paper web and can impair the printing characteristics.
With apertured roller headboxes the material suspension flows through one or more rollers provided with a plurality of apertures, whereby the flocks can likewise be torn apart. Apertured roller headboxes are superior to hydraulic headboxes, particularly with short fibre lengths and slow speeds of flow and result in a softer formation. They do not, however, achieve the quality of hydraulic headboxes for longer fibres, high material densities and high speeds of flow in the headbox. Moreover, flow disturbances are generated by deflection of the flow in the apertured roller headbox.